Fluorinated polythiophenes with ester side chains for boosting the Voc and efficiency in non-fullerene polymer solar cells

As the power conversion efficiency (PCE) of organic photovoltaics (OPVs) approximates the 19 % threshold, wide bandgap (WBG) polythiophenes (PTs) have gained increasing attention due to their simple chemical structures and ease of synthesis, making them promising candidates for large-scale production. However, the benchmark polymer poly(3-hexylthiophene) (P3HT) is limited by its high-lying HOMO energy level, which restricts the open-circuit voltage ( V OC ) in solar cells. In this study, we introduce a novel series of PT derivatives ( PDC8-T , PDC8-3T , PDC16-3T , and PDC16-3T-2F ) featuring ester side chains designed to fine-tune electronic properties through a streamlined three-step synthesis. Additionally, we incorporated a π-spacer to reduce steric hindrance and elongated alkyl side chains to improve solubility and processability . Compared to P3HT, these PT derivatives demonstrate a significant reduction in HOMO energy levels, lowering by approximately 0.3–0.4 eV. Among them, PDC16-3T-2F —with fluorine atom substitution—achieves the lowest HOMO energy level, induces a coplanar molecular conformation , and enhances polymer aggregation behavior. We evaluated these PT derivatives in inverted non-fullerene bulk-heterojunction (NFA BHJ) OPVs. The PDC8-3T device showed a relatively low PCE of 0.69 %, with a V OC of 0.76 V, a short-circuit current density ( J SC ) of 3.32 mA/cm 2 , and a fill factor ( FF ) of 27.3 %. In contrast, the PDC16-3T-2F device achieved an impressive PCE of 7.21 %, with a V OC of 0.85 V, a J SC of 14.60 mA/cm 2 , and an FF of 58.4 %. This remarkable improvement is attributed to the fluorine substitution, which not only enhances molecular orientation but also downshifts the HOMO energy level and further boosts the V OC . Hence, these molecular design strategies have led to a fibrillar bicontinuous interpenetrating network with optimal nanoscale phase separation within the active layer . • A π-spacer is strategically utilized to reduce steric hindrance . • Elongated alkyl side chain length improves the solubility and processability. • Fluorinated PT derivative with ester side chains possesses the lowest HOMO energy level. • Introducing the fluorine atom induces a coplanar conformation and promotes a beneficial face-on orientation. • The optimal phase separation at the nanoscale within the NFA BHJ blends.